Q&A: Solar Eclipses for Newbies

A total solar eclipse as seen from Australia on November 13, 2012. Total solar eclipses expose the Sun's corona, the outer part of the Sun's atmosphere. The corona is usually too faint to see with the naked eye because the Sun's surface is much brighter. Image courtesy NASA.

On August 21, 2017, darkness will fall on North America. As the moon passes between the Earth and Sun, it will cast a shadow that will sweep coast to coast from Oregon to South Carolina. Barring bad weather or an overcast day, people anywhere in the continental United States will be able to look up and see at least a partially eclipsed sun. But what exactly goes on during solar eclipses? And what's the big deal about them? Carnegie astronomer Erika Nesvold answers these questions—and more.

Have you ever seen an eclipse?

Nesvold: I have seen a couple of lunar eclipses, and at least one partial solar eclipse when I was a child, but I've never seen a total solar eclipse. In fact, the last total solar eclipse that could be seen from the continental United States was in 1979, before I was born! But for Monday's eclipse, I'll be in a small town in Idaho where you can see the total solar eclipse, with a bunch of other astronomers as a part of Boise State University's Exoclipse conference.

What are you looking forward to?

Nesvold: I have enjoyed my previous eclipse-viewing experiences because they give me a really nice sense of the enormous dance playing out over our heads between the Earth, Moon, and Sun. But a total solar eclipse is something special, because it's the only time you can see the Sun's corona with the naked eye. This is exciting to me as an astronomer because I learned about the corona in graduate school but have never seen it. It's also exciting to me as a person because it sounds like it's quite beautiful.

What happens during the eclipse? Why do we see what we see?

Nesvold: During a solar eclipse, the Moon passes between the Sun and the Earth. The Moon blocks the light from the Sun, casting a shadow on the surface of the Earth. If you're standing within the path of totality looking up at the Sun (while wearing eclipse glasses) as the Moon's shadow passes over you, you will see a dark disk (the Moon!) cover more and more of the Sun until all of the Sun's light is blocked. The Moon happens to be just about the same size in the sky as the Sun, so when the Moon is lined up directly between you (on the Earth) and the Sun, the Moon will block all of the Sun's light except the faint, wispy corona. Then the Moon will continue on its way, the shadow will move past you, and the Sun will become visible again.

Nesvold presenting her thesis work at the AAS Division for Planetary Sciences meeting 2013 in Denver. Photo by Henry Throop, courtesy Erika Nesvold.

Why are total solar eclipses such a big deal?

Nesvold: Total solar eclipses are very important for scientists studying the Sun (known as heliophysicists), because they give them a chance to directly observe the Sun's corona. For centuries, heliophysicists have traveled to the path of totality during total solar eclipses so they can make observations of the corona with their scientific instruments. Many cultures also consider solar eclipses to be significant events. The Navajo people, for example, don't eat or drink during a solar eclipse, and they go inside and avoid watching the eclipse to show respect for the Moon and Sun. In general, solar eclipses are interesting because they are rare. While a total solar eclipse occurs somewhere on the Earth's surface about once every 18 months on average, a total solar eclipse can only be seen from a given location (say, your house) every few centuries.

Why can you only see the eclipse from certain places on Earth? What's the difference between a total and partial solar eclipse?

Nesvold: Even though the Moon is large enough in the sky to block the Sun, the Moon's shadow on the Earth will only be about 70 miles across during the eclipse. You have to be standing in that shadow to be able to see the total eclipse. If you're standing near that shadow but not inside it, you'll see a partial eclipse: The Moon will block some of the Sun's light but it won't line up perfectly to block all of the Sun's light. The farther you are from the Moon's shadow, the less the Moon will cover the Sun. All of the continental U.S. will be able to see at least a partial eclipse on Monday.

What is the path of totality?

Nesvold: The shadow that the Moon casts on the Earth's surface moves across the planet as the Moon moves through space. This means that the region of the Earth's surface from which you can see the total eclipse is actually a long line, about 70 miles wide but thousands of miles long. This is called the path of totality. The Moon's shadow moves along the path of totality from west to east, because the Moon orbits from west to east as seen from the Earth.

Viewing from a place like in Washington, D.C. people will see the moon cover about 81% of the sun. That sounds like a lot, but is it?

Nesvold: It does sound like a lot, doesn't it? But the Sun's surface is so bright that even if the Moon blocks 99% of the Sun, the sunlight that remains is 10,000 times brighter than the Sun at totality. It won't be like experiencing night during the day, it will be more like experiencing a cloudy day. That remaining sliver of light is enough to damage your eyes if you stare at it, but there are a few ways to safely observe a partial eclipse. I really recommend that everyone in the continental United States and Canada whose cultural beliefs allow it try to observe the eclipse, even if you can only see the partial eclipse. It's a communal experience that can distract you from your earthly troubles for at least a couple of hours.

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Animation showing the path of totality in red across various regions in the United States. Image Courtesy NASA via GIPHY.

How can people protect their eyes while observing the eclipse?

Nesvold: If you have a pair of eclipse glasses, you can safely look directly at the Sun and watch the Moon take a bite out of the Sun. Remember: Sunglasses are not eclipse glasses! Sunglasses let in too much light, and you can suffer permanent damage to your eyes if you try to look at an eclipse with only sunglasses. If you don't have eclipse glasses, you can still observe the partial eclipse by making a simple pinhole projector, which lets you watch the shadow of the eclipse indirectly. Emily Lakdawalla at The Planetary Society has a nice write-up on how to build a pinhole project with kids: http://www.planetary.org/blogs/emily-lakdawalla/2017/sharing-an-eclipse-with-kids.html. Note that kids are not a required ingredient! Also, if you are thinking about using a telescope or binoculars to view the eclipse outside totality, please make sure to use a solar filter! It's just as important as using eclipse glasses for the naked eye.

What could actually happen if you look directly at the sun?

Nesvold: Nothing good! During a partial eclipse, if you stare at the sliver of the Sun, you might not feel any pain. But there is still enough UV light coming from that piece of the sky to permanently burn your retinas. You might not even notice the damage until later that day, so don't count on being able to look at the Sun until it starts to hurt. People who have viewed past solar eclipses without eye protection have ended up with permanent blind spots in their vision. NASA has a lot of information on eclipse safety: https://eclipse2017.nasa.gov/safety.

If the sun is much larger, why does the moon cover it?

Nesvold: The Sun is much, much larger than the Moon, but the Moon is much closer to us than the Sun is. The distances and sizes are just right so that the Moon's apparent size in the sky is the same as the Sun's apparent size. This is just a nice coincidence, and it wasn't always true! The Moon is actually moving away from the Earth at a rate of around 1.5 inches per year (that's roughly the diameter of a golf ball). In prehistoric times, it was much closer, so it looked larger in the sky and would have blocked much of the corona. Millions of years from now, it will be too small to block the whole Sun during an eclipse.

How long does the eclipse last?

Nesvold: How long the totality phase of a solar eclipse (the part where the Moon completely blocks the Sun) lasts for you is determined by how fast the Moon's shadow passes over wherever you're standing. This speed depends on how fast the Moon is moving through space, how fast the Earth is rotating, and where you are on the Earth. The longest possible time of totality during Monday's eclipse will be around 2 minutes, 40 seconds. The longest total solar eclipses can last over 7 minutes. The partial eclipse phase, where only part of the Sun's surface is blocked by the Moon can last a couple of hours on either side of the totality phase. NASA has a great interactive map where you can click on your location and find out everything about your eclipse: https://eclipse2017.nasa.gov/sites/default/files/interactive_map/index.html.

Other than bringing about some shade for a couple of minutes, what else can happen during an eclipse?

Nesvold:Here's what I've heard but haven't yet experienced: A total solar eclipse is like a sunset in the middle of the day. Without the Sun's light and heat, the temperature drops, and a breeze picks up. It's dark enough to see stars in the sky, but the horizon is lit up like a sunset (because the Earth's atmosphere is reflecting light from beyond the Moon's shadow). Non-human animals who depend on the Sun to tell time think that it's night time: birds fly home to roost, ants head back into their hills, crickets start to chirp. And humans, by all accounts, get very, very excited.

What do you recommend people do during the eclipse? Take notes? Admire it? Take pictures?

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During a solar eclipse, the Moon passes between the Sun and the Earth. Image courtesy NASA via GIPHY.

Nesvold: You do you! If you're interested in astrophotography, this is a great chance to test your skills and try to capture a good image of the corona. Personally, I'd rather not be distracted by equipment during my first total solar eclipse, so I'm going to spend my precious two minutes of totality enjoying the sight of the Sun's corona and the reactions of the crowd around me. If your interests run more towards biology than astronomy, you might want to check out the Life Responds app, which lets you report on animal behavior during the eclipse, which scientists will use for their research: https://www.calacademy.org/citizen-science/solar-eclipse-2017.

Would you recommend people to record or watch the eclipse through their smartphone's camera?

Nesvold: You can photograph the Sun with your smartphone without damaging it, even outside of totality, although you have to be careful to avoid looking at the Sun with your eyes while you're lining up your shot. But I wouldn't recommend it. If you've ever tried to take a picture of the Moon in the night sky with your smartphone, you'll know that it's hard to get a picture that looks as good as the real sight. And if I only have a couple minutes of totality, I'd rather spend that time making memories of the view with the naked eye rather than fiddling with my phone to take a mediocre picture.

Any last minute tips?

Nesvold: If you haven't made plans to be in the path of totality for this eclipse and are starting to regret it, you should know that there will be another total solar eclipse in 2024 where the path of totality will again stretch across the continental United States. Start making your travel plans now!